In the recent significant development of smaller-size and lighter-weight optical equipment, aspherical lenses have been used increasingly. The aspherical lens is advantageous in that aberration of light can readily be corrected and that the number of lenses can be decreased so as to allow reduction in size of the equipment.
Besides the aspherical lenses, other uses of the optical glasses have been developed as well, and there is a demand for an optical glass which is capable of transferring a fine structure of a mold with high precision.
For fabricating an aspherical lens or the like, a glass preform is softened by heating, which is then formed into a desired shape by precision-mold press molding. There are generally two ways of obtaining the preform: one is to cut a piece of glass out of a glass block or bar and process it into a preform, and the other is to drop a molten glass from a distal end of a nozzle so as to obtain a glass preform in the spherical form.
In order to obtain a molded product of a glass by way of precision molding, it is necessary to press-mold the preform under the temperature condition near the deformation point (At). Therefore, when the preform has a higher deformation point (At), the mold coming into contact with the preform will be exposed to a higher temperature, causing the surface of the mold to suffer oxidization and corrosion. This gives rise to the need of maintenance of the mold, hindering mass production at a low cost. Accordingly, it is desired that the optical glass constituting the preform can be molded at a relatively low temperature, or, that it has a low glass transition point (Tg) and/or a low deformation point (At).
As to the glass used for a molded lens, a glass having various optical characteristics suitable for its specific use is demanded. In particular, there is an increasing demand for a glass having a high refractive index, low dispersion, and a low deformation point.
Among the conventional glasses, LaK type and LaF type glasses, for example, would meet the above-described demands. However, many of them have a relatively high deformation point, causing the mold susceptible to degradation, and thus, they are undesirable from the standpoint of durability improvement of the mold.
There has been disclosed a B2O3—La2O3—Y2O3—RO—Li2O type optical glass (where RO represents a divalent metal oxide) which has optical constants including a refractive index (nd) of 1.62 to 1.85 and an Abbe number (νd) of 35 to 65 (Patent Document 1).
However, those with a low glass transition point (Tg) and/or a low deformation point (At) exhibit poor resistance to devitrification. Moreover, there is no specific description about molding.
There is also disclosed a B2O3—La2O3—ZnO—Li2O—Sb2O3 type optical glass which has optical constants including a refractive index (nd) of 1.64 to 1.88 and an Abbe number (νd) of 31 to 55. There is further disclosed a B2O3—Li2O—ZnO—La2O3 type optical glass which has optical constants including a refractive index (nd) of 1.66 to 1.77 and an Abbe number (νd) of 43 to 55 (Patent Documents 2 and 3).
These optical glasses, however, pose the problems which are similar to those of Patent Document 1 described above.
There is also disclosed are B2O3—SiO2—La2O3—Y2O3—Li2O—CaO—ZnO type optical glasses which each have a refractive index (nd) of 1.67 or more, an Abbe number (νd) of 50 or more, and a deformation point of 600° C. or lower (Patent Documents 4 and 5).
These optical glasses, however, each have a high deformation point of 530° C. or higher, causing the mold susceptible to degradation.
There is also disclosed a B2O3—SiO2—La2O3—Li2O—SrO—ZrO2 type optical glass which has a refractive index (nd) of 1.60 to 1.75, an Abbe number (νd) of 50 to 60, and a glass transition temperature of 500° C. or lower (Patent Document 6).
This however contains a rare earth oxide in a large amount (22 to 43 wt. %), and it also contains LiO2 in a relatively large amount for the purposes of lowering the glass transition temperature. Accordingly, although the glass may effectively satisfy the optical constants, it is subject to devitrification.
Also disclosed is a B2O3—La2O3—ZnO2—Gd2O3 type optical glass which has a refractive index (nd) of 1.65 to 1.75 and an Abbe number (νd) of 45 or more (Patent Document 7).
This however contains SiO2 in a large amount of 6% or more, and is lowered in basicity for the purposes of preventing the glass and the mold from fusing together. Thus, the glass transition point (Tg) and the deformation point (At) tend to become high, causing the mold susceptible to degradation.
There is further disclosed a B2O3—SiO2—La2O3—BaO type optical glass which has a refractive index (nd) of 1.63 to 1.80 and an Abbe number (νd) of 45 to 60 (Patent Document 8).
However, the contents of alkali oxides and ZnO, effective to lower the glass transition point (Tg) and the deformation point (At), are small. Accordingly, the deformation point tends to become high, causing the mold susceptible to degradation.